Pot odds are as easy as calculating outs. You just compare your outs or your chance of winning to the size of the pot. If your chance of winning is considerably better than the ratio of the pot size to a bet, then you have good pot odds. If it is lower, then you have bad pot odds. To cite an illustration, when you are in a $1/$2 Holdem game with Jack-Ten facing one opponent on the turn. You got an outside straight draw with a board of 2-5-9-Q, and only the river card left to make it. Any 8 or any King will end this straight for you, so you have 8 outs i.e. four 8's and 4 K's left in the deck and 46 unseen cards left. 8/46 is almost similar to having a 1 in 6 chance of making it. Your only rival bets $2. In case you take a $2 bet, you could win $40. $40/$2 comes to 20, so you really stand to make 20x more if you call - 1/6 higher than 1/20, hence pot odds declare that calling wouldn't be a bad idea.
We should perhaps make clear one thing. By some means, a lot many players want to factor in money they wagered on previous rounds. With the last example, you most likely had already invested a significant portion of that $40 pot. Let's say $10. Does that mean you should play or fold since a major amount of that money you already have in there? $10/$40? We would strongly say No. That's not your money any longer! It's in a pool of money to be given to the ultimate winner. You simpy have no "stake" in that pot. The only stake you might have is entirely mental and has no bearing on hard statistics.
There is so much to knowing and implementing about the odds that one learn it all along the way.